fix: lazy-load DocSearch CSS to reduce inlined stylesheet size

[aeneasr]

Summary

• Extracts ~30KB of DocSearch CSS from the main inlined stylesheet into a separate docsearch.<hash>.css file
• Lazy-loads the DocSearch stylesheet only when the user first interacts with the search button (mouseover/focus/click)
• Reduces inlined CSS per page from ~132KB to ~102KB (~23% reduction)

How it works

The preload-css.js post-build plugin now:

1. Splits the compiled CSS into blocks using a brace-depth parser that handles nested @media rules
2. Detects DocSearch blocks (.DocSearch selectors / --docsearch- variables), keeping mixed :root blocks in main CSS
3. Writes the DocSearch CSS to a separate hashed file
4. Injects a small inline script that attaches mouseover/focus/click listeners to the search button and loads the stylesheet on first interaction

Test Plan

• Run npm run build and verify no errors
• Check that inlined <style> in HTML contains no .DocSearch classes
• Verify docsearch.<hash>.css is present in build/assets/css/
• Open built site, open DevTools Network tab — confirm DocSearch CSS loads only after hovering/clicking the search button
• Confirm search works correctly after interaction

[aeneasr]

Unfortunately, these hacks dont work.

Right now, the question is if it makes sense to even keep the search bar since I heard it doesnt work well, if it just adds bloat to the website and makes it slow.

Or we fix the search and it becomes really good, which is also an option ;)

But the work here isn't mathing, it just creates some ugly visual flakes.